Variable resistance



Julie 9, 1931. REISMAN 1,808,790

VARIABLE RESISTANCE Filed March 28. 1929 Tia. 1..

INVENTOR v rm l Reasmqn I BY v ATTNEYS Patented June 7 9, 1931 UNITED STATES PATENT OFFICE EMIL REISMZAN, OF. BROOKLYN, NEW YORK, ASSIGNOR- TO TECHNIDYNE CORPORATION,

OF NEW YORK, N. Y., A CORPORATION OF NEW YORK VARIABLE RESISTANCE Application filed March 28, 1929. Serial No. 350,478.

This invention relates to variable resistances and more particularly to variable high resistances employing a resistance film.

In a copending application of Lester L.

Jones, Serial No. 193,357, filed May 21, 1927, there is disclosed a variable high resistance employing a very thin resistance film coated on an insulation base, with which a sliding contact cooperates for varying the effective 10 resistance in circuit. Because of the high value of the resistance of the film, which itself is determined by the high maximum value of resistance to be rovided by the unit, it has proven very difficult to provide a gradual initial resistance increase from zero. When the sliding contact leaves the metallic short circuiting terminal, even for just a very slight distance, the initial resistance is apt to be of the order of magnitude of 100 ohms.

For many purposes such a resistor, while otherwise entirely satisfactory, is not usable in the absence of some provision for obtaining a much smaller initial resistance and a much smaller gradual increase therein during the initial movements of the sliding contact.

To improve a resistor of this-general type by overcoming the foregoing difficulty is the primary object of the present invention.

The zero resistance point ordinarily is provided by the use of a metallic short circuiting contact. This may conveniently be formed by depositing or painting a metallic film, such as silver, over one terminal of the resistance film. However, the resulting metallic coating tends to wear away rapidly at the edge up onto which the sliding contact rides when the resistance is reduced to zero. Another object of the present invention is to prevent this wearing away of the metallic terminal. a

I fulfill both of the foregoing objects by the relatively simple expedient of inverting the mode of bringing the metallic and resistance coatings into contact. In accordance with my invention the metallic coating is painted directl on the insulation base, and the resistance lm is painted on the insulation base, and then up onto the metallic terminal for a considerable portion of its dis tance. In this manner the s iding contact rides from the resistance film downwardly onto the metallic terminal, and therefore does not tend to wear its edge away. At the same time a gradual initial increase of resistance is provided because the first resistance introduced is that of the thickness of the resistance film, which rovides a very low resistance to current owing transversely of the film, and this resistance is gradually increased as the sliding contact begins to make effective some of the resistance film in a longitudinal instead of in a transverse direction. In one aspect it may be said that the resistance element employed in my invention consists first of a short circuiting metallic terminal, then of a resistance film painted on a highly conductive base, and finally of a resistance film painted on an insulation base.

To the accomplishment of the foregoing and such other objects as will hereinafter appear, my invention consists in the resistor elements and their relation one to the other, as hereinafter are more particularly described in the specification and sought to be defined in the claims. The specification is accompanied by drawings in which:

Fig. 1 is a rear elevation of a complete resistor unit;

Fig. 2 is an enlarged section taken in the plane of the line 22in Fig. 1, and

Fig. 3 is a fragmentary elevation of a modification.

Referring to the drawings it will be seen that the variable high resistance comprises an insulation base 2, a resistance film 4 coated thereon, a sliding contact 6 for cooperation therewith, and a short circuiting terminal 8 at one end of the resistance film.

The base 2 may most simply though not necessarily be provided by stamping a metallic plate 10 to the desired configuration, including the proper holes, and thereafter coating the metallic unit with a baked enamel coating, 12, which forms a hard smooth con tinuous layer. Glass or pyrex also are suitable matcrials from which to form the base.

The resistance variation is obtained by rel ative movement of the film' and a contact, preferably by the provision of a rotatable shaft 14, to which there is affixed a resilient of individual flexible contact fingers, and

the contact arm 16 is flexed in order to provide the desired pressure forbringing the fingers of the sliding contact 6 into intimate contact with the resistance film 4. No claim is laid herein to the structure of the sliding contact, for this is disclosed and claimed in a copending application of Lester L. Jones, Serial No. 350,731, filed concurrently herewit The terminal 8 is best made by depositing or painting a metallic coating on the resistor base at the zero resistance end. The most suitable material is silver, because of its high conductivity, and because it does not oxidize. A mixture of colloidal silver in oil may be painted on the base, and thereafter baked to drive off the oil and fuse the silver together. Now, it has heretofore been customary to apply this silver coating after the resistance film has'been painted upon the base, so that the metallic film is brought into contact with the resistance film by being superimposed thereon. In operation the sliding contact upon leaving the metallic terminal abruptly presents a portion of resistance film between the contact and the terminal which, owing to the high specific resistivity of the film, is ordinarily of the order of magnitude of one or several hundred ohms. Furthermore, the edge of the silver coating tends to wear away because'of the abrasion of the sliding contact'against the raised edge thereof as the sliding contact leaves the resistance film and rides up onto the metal.

Expedients such as a tapered resistance film and others described in a copending application of Lester L. Jones, Serial No. 350,7 31, filed concurrently herewith, may be used to obtain a low initial resistance, in some cases of the order of magnitude of ten ohms. For certain purposes even this is far too high, and an initial resistance as low as a half or one ohm is required.

Now, in accordance with my invention, the metallic terminal 8 is coated directly on the insulation base 2-, and the resistance film 4 is thereafter painted on the base 2 and up over the metallic terminal 8, as is best shown in Fig. 2. In this manner the resistance is very gradually increased because the sliding contact, when it first rides up onto the resistance film, causes a current flow transversely rather than longitudinally through the film. 'When the sliding contact moves away from a posi- 65 tion directly over the silver, the resistance increases only slowly because at first there is a decreased effective area of contact, inasmuch as the advancing edge of the silver contact is moved past the silver terminali As the following edgeof the sliding contact first leaves from directly over the silver terminal it engages the relatively thick portion of the resistance paint, indicated at 20, which fills in the edge of the metal, so that the change in resistance value equivalent to what would be the initial resistance value were the terminal applied over the resistance film, is reached through a number of preliminary stages of lower resistance. In fact, the present resistor is readily capable of providing an initial resistance of the orderof a half or one ohm.

It should be noted that the modification shown in Fig. 1 differs from that shown in Fig. 3 in that in the former the edge 28 of the metallic film is run at a slant, rather than radially with respect to the axis of the unit, so that the fingers of the sliding contact 6 leave that portion of the resistance film which is above the metallic film successively rather than simultaneously. In this manner the effective cross-sectional area of the transverse current path from the contact throughthe resistance film and into the metallic film is gradually diminished. This aids greatly in obtainingthe desired slow growth in resistance above the initial half ohm resistance value. I

It should be understood that the arrangement shown in Fig. 3 also causes a very low resistance value, say a half ohm, so that if other means are provided in the resistance unit for thereafter obtaining agradual increase in resistance, the expedient of placing the resistance film over the metallic film may alone be used merely for the purpose of obtaining the initial low resistance value.

Meanwhile, there isno tendency for the sliding contact 6 to wear away the edge of the metal, because it rides downwardly onto the metal.

It will be noted that the inventionmay be considered as placing between a short circuit terminal and a resistance film an intermediate connecting unit consisting of a resistance film painted on a metallic or highly conductive base. In other words, the resistance element of the present resistor consists successively of a metallic portion, a resistance film coated on a highly conductive base, and finally, a'resistance film coated on an insulation base.

It will be apparent that while I have shown and described my invention in the preferred forms, many changes and modifications may be niade in the structure disclosed without departing from the spirit of the invention, defined in the following claims.

I claim: I s

1. A variable resistance unit comprising a resistance element and a sliding contact in cooperation therewith, said resistance element comprising, successively, a short circuiting metallic terminal surface, a resistance film coated on a conductive base, and a resistance film coated on an insulation base, over each of which the contact may slide.

2. A variable resistance unit comprising a resistance element and a sliding cgntact in cooperation therewith, said resistance element comprising a resistance film coated on an insulation base, a metallic short circuiting terminal therefor, and a resistance film coated on a conductive base arranged between the aforesaid resistance film a nd the terminal in order to provide a gradual initial resistance increase the contact being slidable over each of said portions of the resistance element.

3. A resistor comprising a resistance film, and a painted metallic terminal therefor, said resistance film being placed in contact with the metallic terminal by being coated thereover. l

4. A variable resistor comprising a metallic short circuiting terminal, a resistance film,

and a movable contact in cooperation with said terminal and film, said resistance film being placed in contact with the metallic terminal by being coated over a. portion thereof.

5. A variable resistor comprising a metallic short circuiting terminal of tapered width, a resistance film, and a'movable contact in cooperation therewith, said resistance film being placed in contact with the ta red portion 0 the metallic terminal by being coated thereover.

6. A variable resistor comprising a base, a resistance film thereon, a movable sliding contact therefor, a metallic short circuiting terminal for the resistance film, and means for providing a gradual initial increase in resistance'and for preventing wear of the metallic terminal comprising an extension of the resistance film coated on top of a portion of the short circuiting terminal.

7. A variable resistor comprising an insulation base, a resistance film thereon, a mov able sliding contact therefor, and means for providing a gradual initial increase in resistance comprising a metallic short circuiting terminal for the resistance film, having an inner portion of gradually diminishing width, and an extension of the resistance film painted on top of the inner portion of the short circuiting terminal.

8. A variable resistor comprising a base, a resistance film-thereon, a movable sliding contact therefor, a painted silver short circuiting terminal for the resistance film, and means for providing a gradual initial in crease in resistance and for preventing wear of the silver terminal comprising an extension of the resistance film coated on top of a portion of the short circuiting terminal.

thereto, a painted silver short circuiting ter-v minal for the resistance film, and means for providing a gradual initial increase in resistance and for preventing wear of the silver terminal comprising an extension of the resistance film painted on top of a portion of the short circuitin terminal.

Signed at New ork, in the county of New York and State of New York, this 25th day of March, A. D. 1929.

EMIL REISMAN.

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